Compressors are widely used in turbomachinery applications to compress an intake airflow. It is known to provide the airflow entering the front stages of the compressor with a specific axial orientation in order to maximise the efficiency of the compressor and/or to provide an adequate stability margin.
This airflow orientation is determined by the stagger angle of the guide vanes directing air onto the rotor stages of the compressor.
The compressor blading is designed to deliver optimum performance at a design point and therefore, changes in rotational speed and air mass flow during operation away from the design point result in changes in airflow velocity components which, in turn, penalises off-design performance.
Since variable operating conditions are encountered during operation of a typical gas turbine engine, it is known to use variable guide vanes which may be rotated around an axis so varying the stagger to meet engine operability requirements.
A conventional compressor is typically equipped with variable inlet guide vanes as well as several rows of variable stator vanes that each redirect the airflow by a rigid rotation.
A problem with such arrangements is that when they are rotated at off-design conditions, the airflow may separate along the blade surfaces.
For example, for a variable inlet guide vane it can be assumed that the incoming airflow has approximately the same flow angle at every operating condition. At the design point, the variable inlet guide vanes are aligned to the incoming airflow because redirecting the flow is not necessary. However, away from the design point (i.e. at part speed conditions) the variable inlet guide vane is rotated to achieve the outlet flow angle that would result in an acceptable rotor blade incidence. Since the variable inlet guide vane rotates rigidly while the incoming airflow direction does not change, the airflow incidence at the variable inlet guide vane inlet increases. This can cause air flow separation with a consequent increase in pressure loss.
It is known to provide the variable inlet guide vane with a constant leading edge angle and a variable trailing edge angle using a hinged or ‘variable camber line’ configuration. One such approach uses a tandem aerofoil design to achieve this, with such geometry providing a sensible reduction in pressure losses.
A known disadvantage with this approach is that such tandem aerofoil arrangements are susceptible to flow separation on the moving part of the aerofoil at high turning angles. This is due to the fact that tandem aerofoil profiles have a geometrical discontinuity that can trigger flow separation. This flow separation is particularly pronounced at operating conditions away from the design point because this necessitates high turning angles which cause this discontinuity to be more pronounced.